In the field of optical transmission, devices for generating a modulation of an optical signal are used. Throughout the remainder of the description, a device for generating a modulation of an optical signal will simply be referred to as a modulation device.
When a modulation device is used to perform an intensity modulation of an input optical signal, the output optical signal obtained is generally also phase modulated, which is not desired. By definition, the chirp parameter denoted as a is reflective of the existence of this undesirable phase modulation over the output signal. A possible mathematical definition of the chirp parameter α is as follows:
  α  =      2    ⁢                  I        ⁡                  (          t          )                    .                                    ⅆ                          ϕ              ⁡                              (                t                )                                                          ⅆ            t                                                ⅆ                          I              ⁡                              (                t                )                                                          ⅆ            t                              
where:                I(t) is the instantaneous intensity of the optical field measured at an output of the modulation device considered, and        Φ(t) is the instantaneous phase of the optical field measured at an output of the modulation device considered.        
In the monograph by Mr. Seimetz which is entitled “High-Order Modulation for Optical Fiber Transmission” that was published by Springer in 2009, it is shown how to generate optical signals following a complex modulation. Such a complex modulation is referred to as “I-Q modulation” (I is the “in-phase” component, and Q the quadrature component). The generation of optical signals following a complex modulation involves the use of several modulation devices having a zero chirp parameter α.